The main reason most folks can get away with training with no tsuba is that they do not really go for it. If you are doing a per-arranged form in which everyone knows exactly what is going to happen, and the parties are trying to do the form rather than really hit each other, you can have a very controlled exercise and no ones hands get clobbered..

Dear Ledyard sensei

This is not necessarily contradicting your point but I'd just like add for the benefit of others that in some pre-arranged forms such as the kumi-tachi of the founder and his closer students (most notably Saito Morihiro Shihan), the practitioners actually are "trying to hit each other" once both parties know the form well enough for it to be safe. The forms actually fall apart if one merely attacks the air in front of one's opponent or if one changes the target from a strike to the yokomen to a strike against the sword.

So I agree with you regarding when the form demands a particular target such as the sinciput of the head rather than the hand. Some henka do target the hand, wrist or forearm, but as you say, since it's a set form, you know the person will try to hit that area, so you have a chance to build up to intense practice without a tsuba. Outside of the form, especially in competition, it is a different matter.

I always ask myself what the sword practice is trying to ingrain. In one dojo I visited they didn't use tsuba but beginners had to wear a hockey glove until they learned that particular form. In one of their kata, the hand was a specific target from a specific stance that seemed to cultivate a particular Iaido-related reaction in uketachi. Another dojo I visited practised with and without the tsuba, but insisted that bokken without tsuba be gripped as if they had one. In other words, if the teacher came over and put a shinken in your hands, your grip would be the same with the real sword (with tsuba) as it was with the (no-tsuba) bokken. This greatly affected the "shibori" (wringing out) tanren for the cuts and to me it felt like a much more "sword-specific" practice. I'm not saying that's a good or bad thing, it just depends on what the teacher is trying to achieve.

Regards

Carl

PS: I can't immediately recall myself if there are any pictures or videos of Osensei practising with a real katana. It would be interesting to see how he held it.